1. Field of the Invention
This invention relates to an image combination device which combines at least two images with their different light exposures to obtain wide dynamic range image with corrected luminance level.
2. Description of the Related Art
Generally, in a solid-state image sensing device such a video camera or digital still camera, the dynamic range can not be enhanced efficiently, so that in the case of photographing in backlight or remarkable contrast, the background for an object becomes canescent if the light exposure is matched for the object and the object becomes black if the light exposure is matched for the background. As a result, the photographing process can not be performed efficiently.
In this point of view, it is proposed to photograph plural images with their respective different light exposures, to define the too bright images and too dark images and to substitute the too bright images and too dark images with images of the appropriate light exposures to obtain a wide dynamic range image.
In this case, however, the switching level for the too bright images and too dark image is defined uniformly, so that a psued-profile or an unnatural combined boundary is created in the resultant combined image. As a result, the quality of the combined image may be deteriorated. In this point of view, the switching level is an important factor in the high precise creation of the combined image.
In Japanese patent Application laid-open 7-131708, such an image combination device is disclosed as to calculate thresholds for too bright regions and too dark regions through the brightness level control calculation at every combination of plural images with different their respective light exposures from an imaging device, and to substitute the too bright regions and too dark regions with non-standard images on the thresholds. In this case, the psued-profile, which is originated from ununiform brightness level due to the fluctuation of the illumination intensity, can be repressed in the resultant combined image.
In Japanese patent Application laid-open 5-130499, such an image combination device is disclosed as to obtain a first image information and a second image information which are photographed with their respective different light exposures to make a brightness histogram on the first image information, to set the minimum bright level on the brightness histogram as a substitution standard brightness level, and to calculate the brightness average of the second image information. In this case, if some white areas are created on the brightness average, the pixels with brightness more than the standard brightness level are substituted, and if some black areas are created on the brightness average, the pixels with brightness less than the standard brightness level are substituted.